1. Field of the Invention
The present disclosure relates to a large area organic light emitting diode display, and more particularly, to an organic light emitting diode display having an auxiliary cathode electrode for lowering the surface resistance of the cathode electrode and a protective electrode for preventing the auxiliary cathode electrode.
2. Discussion of the Related Art
Nowadays, various flat panel display devices are developed for overcoming many drawbacks of the cathode ray tube such as heavy weight and bulk volume. The flat panel display devices include the liquid crystal display device (or LCD), the field emission display (or FED), the plasma display panel (or PDP) and the electroluminescence device (or EL).
FIG. 1 is a plane view illustrating the structure of the organic light emitting diode display having the active switching elements such as the thin film transistors according to the related art. FIG. 2 is a cross sectional view illustrating the structure of the organic light emitting diode display along to the cutting line of I-I′ in FIG. 1 according to the related art.
Referring to FIGS. 1 and 2, the organic light emitting diode display comprises a thin film transistor (or ‘TFT’) substrate having the thin film transistors ST and DT and an organic light emitting diode OD connected to and driven by the thin film transistors ST and DT, and a cap ENC joining and facing the TFT substrate with an organic adhesive POLY therebetween. The TFT substrate includes a switching thin film transistor ST, a driving thin film transistor DT connected to the switching thin film transistor ST, and an organic light emitting diode OD connected to the driving thin film transistor DT.
On a transparent substrate SUB, the switching thin film transistor ST is formed where a gate line GL and a data line DL are crossing each other. The switching thin film transistor ST acts for selecting the pixel which is connected to the switching thin film transistor ST. The switching thin film transistor ST includes a gate electrode SG branching from the gate line GL, a semiconductor channel layer SA overlapping with the gate electrode SG, a source electrode SS and a drain electrode SD. The driving thin film transistor DT acts for driving an anode electrode ANO of the organic light emitting diode OD disposed at the pixel selected by the switching thin film transistor ST. The driving thin film transistor DT includes a gate electrode DG connected to the drain electrode SD of the switching thin film transistor ST, a semiconductor channel layer DA, a source electrode DS connected to the driving current line VDD, and a drain electrode DD. The drain electrode DD of the driving thin film transistor DT is connected to the anode electrode ANO of the organic light emitting diode OD.
As one example, FIG. 2 shows the thin film transistor of top gate structure. In this case, the semiconductor channel layers SA and DA of the switching thin film transistor ST and the driving thin film transistor DT are firstly formed on the substrate SUB and the gate insulating layer GI covers them and then the gate electrodes SG and DG are formed thereon by overlapping with the center portion of the semiconductor channel layers SA and DA. After that, at both sides of the semiconductor channel layers SA and DA, the source electrodes SS and DS and the drain electrodes SD and DD are connected thereto through contact holes penetrating an insulating layer IN. The source electrodes SS and DS and the drain electrodes SD and DD are formed on the insulating layer IN.
In addition, at the outer area surrounding the display area where the pixel area is disposed, a gate pad GP formed at one end of the gate line GL, a data pad DP formed at one end of the data line DL, and a driving current pad VDP formed at one end of the driving current line VDD are arrayed. A passivation layer PAS is disposed to cover the upper whole surface of the substrate SUB having the switching and the driving thin film transistors ST and DT. After that, formed are the contact holes exposing the gate pad GP, the data pad DP, the driving current pad VDP and the drain electrode DD of the driving thin film transistor DD. Over the display area within the substrate SUB, a planar layer PL is coated. The planar layer PL makes the roughness of the upper surface of the substrate SUB in much smoother condition, for coating the organic materials composing the organic light emitting diode on the smooth and planar surface condition of the substrate SUB.
On the planar layer PL, the anode electrode ANO is formed to connect the drain electrode DD of the driving thin film transistor DT through one of the contact holes. On the other hands, at the outer area of the display area not having the planar layer PL, formed are a gate pad electrode GPT, a data pad electrode DPT and a driving current electrode VDPT connected to the gate pad GP, the data pad DP and the driving current pad VDP, respectively, exposed through the contact holes. On the substrate SUB, a bank BA is formed covering the display area, excepting the pixel area.
On the bank BA and the exposed anode electrode ANO by the bank BA, an organic light emission layer OL is stacked. Then, on the organic light emission layer OL, a cathode electrode CAT is deposited. As a result, an organic light emitting diode OLED having the stacked structure of the anode electrode ANO, the organic light emission layer OL and the cathode electrode CAT is completed.
A cap TS is joining the thin film transistor substrate having above mentioned structure with the constant gap therebetween. In that case, it is preferable that the thin film transistor substrate and the cap TS are completely sealed by having an organic adhesive FS between them. The organic adhesive FS prevents moisture and gases from intruding into the inner space of the thin film transistor substrate. The gate pad electrode GPT and the data pad electrode DPT exposing to the exterior of the cap ENC may be connected to external devices via the various connecting means.
In addition, the cap TS includes a black matrix BM disposed at the non-display area and a color filter CF disposed at the display area, on the inner side of the cap TS. Especially, in the case that the organic light emission layer OL generates the white light, the full color including red(R)-green(G)-blue(B) colors can be represented by using the color filter CF.
For the organic light emitting diode display having such a structure mentioned above, the cathode electrode CAT supplied with the reference voltage is deposited over the whole surface of the substrate for the display panel. When the cathode electrode CAT is made of metal material having relatively lower resistance, there is no problem. When the cathode electrode CAT is made of a transparent conductive material for ensuring enough transmittivity, the surface resistance of the cathode electrode CAT is high so this may cause degradation of the video quality.
For examples, when the cathode electrode CAT includes a transparent conductive material such as the indium-tin-oxide or the indium-zinc-oxide having higher resistivity (or, specific resistance) than metal materials, the surface resistance of the cathode electrode CAT is relatively high. As a result, the cathode electrode CAT may not have even voltage distribution over the whole surface of the display panel. This may cause the unevenness of the brightness or luminance of the display panel. Especially, as the area of the organic light emitting diode display is getting larger, the unevenness of the luminance or lightness may be severely caused.